In the semiconductor industry, ion implantation systems are typically employed to dope a workpiece with impurities. In such systems, an ion source ionizes a desired dopant element, wherein ions are generally extracted from the ion source in the form of an undifferentiated ion beam. The undifferentiated ion beam is typically directed into a beamline assembly comprising a mass analysis apparatus or mass analyzer, wherein ions of a desired charge-to-mass ratio are selected using magnetic fields. Mass analyzers typically employ a mass analysis magnet (also called an AMU magnet) to create a dipole magnetic field, wherein various ions in an ion beam are deflected via magnetic deflection in an arcuate passageway that effectively separates ions of different charge-to-mass ratios. The mass of an ion relative to the charge thereon (i.e., the charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the selected or desired ion beam can be made very pure, since ions of undesirable molecular weight will be deflected to positions away from the beam. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis.
The selected or desired ions are then directed at a surface of the workpiece positioned in a target chamber, wherein the workpiece, (e.g., a semiconductor wafer) is generally implanted with the dopant element. Accordingly, the ions of the desired ion beam penetrate the surface of the workpiece to form a region having a desired characteristic, such as a desired electrical conductivity useful in the fabrication of transistor devices.
The ion beam may be a spot beam (e.g., a pencil beam), wherein the workpiece is mechanically scanned in two dimensions orthogonal to the generally stationary spot beam; a ribbon beam, wherein the beam is formed or electromagnetically scanned in one direction across the workpiece while the workpiece is mechanically scanned in an orthogonal direction; or an electromagnetically scanned beam that is electromagnetically scanned in two directions across a stationary workpiece. Examples of ion implantation systems include those available from Axcelis Technologies of Beverly, Mass.
The ion beam may be further focused and directed in front of the desired surface region of the workpiece in the target station, wherein the energetic ions of the ion beam may be accelerated or decelerated to a predetermined energy level to properly penetrate into the workpiece. The ions, for example, are embedded into a crystalline lattice of the material to form the region of desired conductivity, with the energy of the ion beam generally determining the depth of implantation.
In the semiconductor industry, workpieces are commonly implanted with ions via so-called high current and/or low current ion implanters. In high current implanters, for instance, ion beams having a high beam current ranging between tens of KeV to lower hundreds of eV are commonly provided. In such high current implanters, the ion beam is typically difficult to focus due to the space charge of the low energy associated therewith. One solution has been to provide an additional focusing force through the use of optics associated with the extraction of ions from the ion source. However, the use of ion source extraction optics, alone, has proved to be insufficient in providing the necessary focusing force for the high current ion beam along the entire beamline, and additional focusing is typically needed to achieve an acceptable ion implantation. Furthermore, high current ion implanters typically operate in a deceleration mode with an energy filter, wherein the AMU magnet is “de-tuned”, thus resulting in a shifting of the optical focal point of the ion beam.
Thus, it is desirable to provide an apparatus and method for controlling the focal point of the ion beam, in order to return the focal point of the ion beam to its original position.